I tried to follow the HOWTO to set up xorg.conf manually but the strange thig is that I don't have the /etc/X11/xorg.conf.d/ directory. There is only Sessions, xinit directories and chooser.sh and startDM.sh. I only can find /usr/share/X11/xorg.conf.d/ and that contains 10-evdev.conf and 50-synaptics.conf.

I'm actually using a similar hardware as you (Optimus, e.g. intel + nvidia) on my Arch Linux laptop. The easiest solution for me to get it to work was to remove nouveau and intel drivers and go for the nvidia blob just as billium suggests.

The big problem with nouveau for me is to get hardware acceleration to work. Currently, gnome-shell is consuming 300% CPU (I think it is hw acceleration, filed a bug here: https://bugzilla.gnome.org/show_bug.cgi?id=686218), but with nvidia blob this is not the case. Also, I had to struggle to get vga_switcheroo to work as it should...

for a start add intel to VIDEO_CARDS, emerge xf86-video-intel and make sure the correct kernel bits for intel are selected too.

That did the trick. I added intel to the VIDEO_CARDS in /etc/portage/make.conf, emerged xf86-video-intel and re-emerged xorg-server. If I now run startx I get twm. So I can continue. No idea what driver it is using richt now. But if I have some desktop I'm happy.